The present invention relates to a display device, and in particular, to a display device having a high frequency driving device suitable to display motion pictures with super high definition.
Thickness and weight of picture display devices have been decreased these days, and flat-panel displays (FPD) such as a liquid-crystal display, a plasma display panel (PDP), an electroluminescent (EL) display, and the like have been widely employed in place of a cathode-ray tube (CRT) primarily used as the picture display device. Techniques for a field emission display (FED) and the like are also rapidly being developed. Since personal computers, digital versatile discs, and digital broadcasting are broadly developed, it is required to display super high definition high-speed motion pictures. Requirement for higher performance of the picture display device, particularly, requirement for higher definition and speed of pictures will be needed also in future. In this connection, a liquid-crystal display going ahead of a flat panel display (FDP) is highly expected.
Description will be given of a thin-film transistor (TFT) active matrix driving method as a typical liquid-crystal display driving method of the background art. A TFT active matrix liquid-crystal display is driven in a line sequential scanning method in which a scanning pulse is applied to each scanning electrode for each frame period of time. The frame period of time is usually set to about {fraction (1/60)} second (s). The pulse is ordinarily applied in a direction from an upper side of the panel to a lower side thereof by sequentially shifting timing of the pulse. Therefore, in a liquid-crystal display configured as 1024 subpixels by 768 subpixels, 768 gate interconnection lines are scanned for each frame. Time width of the scanning pulse is consequently about 20 microseconds (μs)≈({fraction (1/60)})×({fraction (1/768)}) s.
On the other hand, at timing synchronized with the scanning pulse, a liquid-crystal driving voltage is applied at a time to signal electrodes for liquid crystal of one row to which the scanning pulse is applied. In one of the selected subpixels to which the gate pulse is applied, a gate electrode voltage of a thin film transistor (TFT) connected to the scanning electrode becomes higher to turn the transistor on. In this state, the liquid-crystal driving voltage is applied via a region between a source and a drain of the transistor to a display electrode. As a result, subpixel capacity including liquid-crystal capacity between the display electrode and an opposing electrode formed on an opposing substrate and load capacity of a load on the subpixel is charged during the period of 20 μs described above. By repeatedly conducting the operation, the liquid-crystal apply voltage is repeatedly applied to subpixel capacity of the overall panel for each frame period of time.
The display device of the background art is operated in the TFT active matrix driving method as above. Therefore, with increase in the number of subpixels for higher definition display, the width of time of the scanning pulse becomes shorter. That is, the subpixel capacity must be charged during a short period of time. Additionally, to display high-speed motion pictures, it is required to reduce one frame period of time. This also minimizes the time width of the scanning pulse.
In other words, the image display methods and the image display driving methods of the background art are attended with delay of signals on interconnection lines, insufficient time to apply data in each subpixel, increase in the scanning frequency, and the like. Therefore, it is difficult to cope with the increase in the display frequency to display a picture with higher definition.
Deterioration in quality of a motion picture displayed on a hold emission display device such as a liquid-crystal display is described, for example, in pages 19 to 26 of Technical Report of the Institute of Electronics, Information and Communication Engineers EID96-4 (1996-06). According to EID96-4, the eyes of a human watching the motion picture can appropriately follow a motion picture produced by hold emission. Therefore, this causes blur in the motion picture to resultantly reduce the picture quality. The article also describes a method to improve the deteriorated picture quality of the motion picture, for example, a method in which the frame frequency is multiplied by n. In the method, the display frequency is increased when a motion picture is clearly displayed on a hold emission image display device such as a liquid-crystal display. However, the display frequency is approaching its upper limit in the image display method and the image display driving methods at the present stage of the technique as already described above.
To cope with increasing requirements for high definition display of motion pictures, new materials have been discussed to reduce interconnection resistance and interconnection capacity, which are factors to delay signals on the interconnection line. To increase performance of writing data in subpixels, a TFT using polycrystalline silicon in place of the background-art TFT using amorphous silicon has been recently put to the market.
JP-A-08-006526 describes a liquid-crystal display device including a unit to conduct change-over between one-line selection and multi-line simultaneous selection to thereby change resolution. However, the resolution is fixed for each line in the technique. The article does not described any method of achieving both of the high definition display and the high-speed display. JP-A-09-329807 describes a liquid-crystal display device including a block selecting unit to save power consumption. The device conducts a re-writing operation in a block unit only for an image of which the contents have been re-written. However, during the motion picture display operation in which the overall screen is re-written, high-speed display of a motion picture is difficult due to the delay of signals on the interconnection line and the restricted performance of data writing operation.
Description will now be given of transmission of an image from an image controller (a graphic controller board) for high definition display and high-speed display to an image display device. Assume that the image display device is, for example, a liquid-crystal display of the background art including a display screen of “1204×768 subpixels”; each of red, green, and blue is represented by eight bits (for about 1.6 million colors), and the frame frequency is 60 Herz (Hz). The bit rate is then about 1.1 gigabits per second (Gbps). Such data cannot be transferred through one data line. To overcome the difficulty, 24 data lines are used to reduce the bit rate of each data line to transmit data to the liquid-crystal display panel. In short, because of the increase in the number of pixels and the increase in the frequency to cope with the high definition display and the high-speed display, the image processing in the image controller and the data transmission between the image controller and the image display device become difficult.
To increase the amount of information to be displayed, four problems exist as described above. (1) Improvement of substantial display data transfer performance, (2) increase in processing performance of a data processor in the display device, (3) increase in display performance of the display device, and (4) reduction in the aperture ratio associated with improvement of definition and resolution for displayed images.
For item (1), namely, for the improvement of substantial display data transfer performance, there have been considered a digital PV (Packet Video) link method in which an image is compared with an image of an image of an immediately preceding frame to transfer only data of an image area of which the contents are changed and a method in which an image is compressed such that the compressed image is not perceived by eyes of a human so as to transfer the compressed data as described in page 38 of “SID '00 Digest”.
For item (3), namely, for the increase in display performance of the display device, there exits a display method in which according to the increase in the display frequency, the image is re-written at a high speed to thereby display the image. For example, JP-A-11-075144 describes a display method in which for each subpixel of optical spatial modulating device, two memories, i.e. first and second memories and a driving unit to drive the subpixel according to the contents of the memories are disposed. For all pixels of an image to be displayed, data is written in the first memory in advance. Thereafter, data is transferred at a time to all subpixels of the second memory. According to the data in the second memory, the driving unit controls a state, namely, on or off of light in each pixel at a high speed to display a multi-level image by pulse width modulation (PWM).
However, when a display device of the background art receives data in the PV link method or the image compression method, the display device cannot directly display the received image data. Therefore, the second problem must be solved, namely, the increase in processing performance of a data processor in the display device must be achieved. Moreover, since nothing has been conducted for item (3), the image cannot be normally displayed.
When the method described in JP-A-11-075144 is employed for item (3), the display data thus received cannot be directly displayed because pulse width modulation (PWM) is used in the multi-level display method. Therefore, it is required to enhance item (2), namely, to further increase the processing performance. Increase in the processing circuit considerably increases the cost.
For item (4), namely, the reduction in the aperture ratio associated with improvement of definition and resolution for displayed images, the driving method of the background art has discussed in various ways. However, a display method of displaying an image using an image compression method has not been discussed at all.